In an image forming device of an electrophotographic system (including an electrostatic recording system), such as a copying machine, laser beam printer or facsimile, a developer is used for making an electrostatic latent image formed on a photosensitive member visible. The developer comprises, as a main component, colored resin particles (toner) with a colorant, a charge control agent, a wax and the like dispersed in a binder resin.
The toner is roughly divided into a pulverized toner obtained by a pulverization process and a polymerized toner obtained by a polymerization process. In the pulverization process, a pulverized toner is obtained as colored resin powder by a process, in which a thermoplastic resin is melted and kneaded together with additive components such as a colorant, a charge control agent and a wax, and the resultant kneaded product is pulverized and classified. The thermoplastic resin used in the pulverization process is synthesized by polymerizing a polymerizable monomer in advance. On the other hand, in the polymerization process, a polymerized toner is obtained as colored resin particles by a process, in which a polymerizable monomer composition containing a polymerizable monomer and additive components is polymerized in the presence of a polymerization initiator in an aqueous dispersion medium.
Even in any process, it is difficult to completely polymerize the polymerizable monomer in a polymerization step, and it is inevitable that an unreacted polymerizable monomer remains in the toner. The unreacted polymerizable monomer remaining in the toner (hereinafter also referred to as “the remaining monomer”) causes such various problems that (1) the remaining monomer is vaporized out of the toner by heating upon fixing, or the like to worsen a working environment or emit offensive odor, (2) the toner undergoes blocking during its storage, (3) the flowability of the toner is deteriorated to lower the quality of an image formed with such a toner, (4) the toner is easy to cause offset, and (5) the toner is easy to cause toner filming on individual members in an image forming device even though the amount thereof is slight.
The problems caused by the remaining monomer are more serious in the polymerized toner than in the pulverized toner. In the case of the pulverized toner, the content of the remaining monomer is easily reduced by a heat treatment and a drying treatment in a preparation stage and a melting and kneading stage of the thermoplastic resin which will becomes a binder resin. On the other hand, in case of the polymerized toner, the remaining monomer must be removed from the polymer particles containing the additive components such as the colorant, charge control agent and wax. However, the remaining monomer is easy to be absorbed in these additive components, so that it is difficult to reduce the content of the remaining monomer compared with the case of the binder resin alone. In addition, the polymerized toner is easy to aggregate or fuse, so that there is a limit to the demonomer treatment by the heat treatment or the like. In recent years, there has been a strong demand for development of a polymerized toner capable of being fixed at a low temperature for the purpose of achieving speeding-up of printing and formation of full-color images. However, it is extremely difficult in such a low-temperature fixing toner to reduce the content of the remaining monomer while preventing aggregation or fusion.
The polymerized toner contains various volatile organic compounds secondarily produced in the polymerization reaction in addition to the remaining monomer. The volatile organic compounds are vaporized out of the toner by heating upon fixing, or the like, so that the volatile organic compounds have an important adverse influence on the toner properties like the remaining monomer. In particular, when a polymerization initiator high in initiation efficiency is used, volatile organic compounds such as ether compounds are easy to be synthesized by side reactions, and such compounds come to be contained in the resulting polymerized toner. The unreacted polymerizable monomer and the other volatile organic compounds will hereinafter be referred to as volatile organic components (VOC) collectively.
As a method for reducing the content of the remaining monomer or volatile organic compounds in the polymerized toner, it has heretofore been known to polymerize a polymerizable monomer composition comprising a polymerizable monomer and a colorant in an aqueous dispersion medium and then subject a dispersion containing polymer particles formed to a stripping treatment.
As a stripping treatment method using saturated steam, for example, Patent Literature 1 discloses a stripping treatment method, in which after a polymerizable monomer composition including a polymerizable monomer and a colorant is suspension-polymerized, a suspension (dispersion) containing toner particles (polymer particles) is heated, and water is then distilled out of the suspension while saturated steam at 100° C. is introduced into the suspension. Patent Literature 2 discloses a stripping treatment method, in which after a polymerizable monomer composition including a polymerizable monomer and a colorant is suspension-polymerized, saturated steam at a temperature higher than 100° C. is introduced into an aqueous medium (dispersion) containing toner particles. Patent Literature 3 discloses a stripping treatment method, in which a carrier gas is introduced into a polymer dispersion containing toner particles to remove organic volatile components (volatile organic components). In Patent Literature 3, saturated steam is shown as the carrier gas.
However, the stripping treatment method using saturated steam of a high temperature by itself takes a long time to reduce the content of the remaining monomer or volatile organic components, and the degree of reduction is not always sufficient, so that this method is not efficient. In addition, when the high temperature saturated steam is blown into the dispersion containing the polymer particles, the polymer particles are easy to cause partial aggregation and fusion. In particular, when the method of continuously blowing the saturated steam of the high temperature exceeding 100° C. is adopted, the temperature of the dispersion is raised even to 100° C. during the stripping, so that the polymer particles designed to be a low-temperature fixing type are easy to fuse or change the surface properties thereof. Further, according to the method of blowing the saturated steam of the high temperature, the liquid level of the dispersion is greatly raised during the long-time stripping treatment, or it is difficult to control the level of a bubble layer on the liquid level. When water is distilled off during the stripping treatment for inhibiting the rise of the liquid level, the heat for distillation is required, and the process thereof becomes complicated.
On the other hand, as a stripping treatment method using an inert gas, for example, Patent Literature 4 discloses a method, in which in a step of subjecting a dispersion containing polymer particles to a stripping treatment to remove the remaining monomer in the polymer particles, a gas such as an inert gas is blown while a bubble level on the liquid level of the dispersion is controlled. Patent Literature 5 discloses a stripping treatment method, in which a gas composed of air or an inert gas is blown into a dispersion containing colored polymer particles (polymerized toner) in an evaporator, and at this time, the flow rate of the gas and the pressure of a vapor phase in the evaporator are controlled within respective specified ranges.
According to the stripping treatment method of blowing the gas such as the inert gas into the dispersion containing the polymer particles, the content of the remaining monomer can be reduced while preventing the aggregation and fusion of the polymer particles. In addition, the flow rate of the inert gas is controlled, whereby the level of a bubble layer generated on the liquid level of the dispersion can be controlled.
However, the conventional stripping treatment method using the saturated steam or inert gas by itself is not sufficient to efficiently and sufficiently reduce the content of total volatile organic components including the remaining monomer (TVOC content).
In order to solve this problem, Patent Literature 6 discloses a method in which, in a step of polymerizing a polymerizable monomer composition including a colorant and a polymerizable monomer to obtain a dispersion containing polymer particles and subjecting the dispersion to a stripping treatment to remove volatile organic components including an unreacted polymerizable monomer, both an inert gas and saturated steam are used as a gas blown into the dispersion.